In operation, drainage pumps used in appliances, such as washing machines or dish washers, usually go through a full water phase, where the pump housing of the drainage pump is full of a liquid such as water, and a fluid or air/water phase, where the water in the pump housing is mixed with air. During the air/water phase, air is sucked into the drainage pump, producing an increase in the noise created by the pump.
Traditional pump housings have a smooth inner surface. It is thought that this smooth surface allows for a relatively smooth flow of the fluid within the pump housing or pump chamber, allowing the air/water mixture to contain large bubbles or pockets of air. It is thought that these air pockets are responsible for producing an irritating noise of varying frequency or pitch which is emitted by the drainage pump during the air water phase. This noise is referred to herein as an intermittent noise due to its perceived variability although in fact it may not be truly intermittent.